Structural assemblies suitable for under-floor airframes are designed to carry aircraft flight loads, such as flight deck loads, passenger compartment loads and cargo hold loads, and are typically equipped with additional energy-absorbing systems, structures or mechanisms for absorbing energy under compression loading, such as during hard landing or crash situations. For example, in vertical-lift aircraft (i.e., rotorcraft), which generally have a shallow under-floor depth, are subject to high vibratory loads, and may be subjected to significant vertical impact during hard landing or crash situations, the following solutions have been employed for energy absorption: (i) deployable/extendable energy-absorbing devices; (ii) vehicle-level air bag systems; and (iii) modifications to airframe structures, such as structures featuring energy-absorbing tubes, sine wave shaped beams, corrugated shaped beams, conosoid shaped beams and honeycomb core structures. Such energy-absorbing systems, structures or mechanisms add parasitic weight to an aircraft, present space and structural integration issues, and have increased production and/or maintenance costs.
The majority of military rotorcraft have metallic under-floor airframe structures comprised of frames, formers and longerons with cross-sections in the shape of an “I”, “J” or “Z” (also referred to as “I-beam,” “J-beam” and “Z-beam” cross-sectional shapes), and which are formed from either sheet metal or high-speed machining of metal billets. Composite materials are increasingly being used in airframe designs for both weight and cost savings. Structures formed of composite material are advantageous due to their high strength-to-weight ratio, favorable corrosion resistance, and high specific energy-absorbing capability during compression-loading events. Compared to metals, composite materials are typically brittle and do not exhibit plasticity or high elongation prior to failure. Composite structures may be formed by laying up composite plies comprised of reinforcing fibers embedded in a polymer matrix. Composite structures are typically designed to transmit loads along the length of the reinforcing fibers. Loads from one fiber may be transferred to another fiber in the same layer or to fibers in an adjacent layer by passing through the matrix material. However, the matrix is typically weaker than the fibers such that when a sufficiently-high load is placed on the composite structure in an out-of-plane direction or in a direction non-parallel to the fibers, the matrix may fail.
In conventional composite structures, the composite plies are typically aligned with and define the outer geometry of the structure. However, a composite structure may be subjected to loads that are oriented non-parallel to the fibers and/or in an out-of-plane direction relative to the plane of the composite plies. Such non-parallel and out-of-plane loads may result in interlaminar tension effects that may exceed the load-carrying capability of the composite structure. To avoid overloading the composite structure, additional composite plies may be required which may increase the weight and complexity of the composite structure.